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Frost Dates

The single most important aspect of the weather that gardeners in temperate climates need to know is when the first and last frosts will occur. Many plants will be killed or severely injured by even a single night of freezing temperatures. Some of these can be started indoors - and for certain plants that either require or benefit from a longer growing season than we normally have in Pennsylvania, it's almost a necessity. But starting them too soon or too late can limit their growth, which is why seed packet instructions recommend sowing within a specific period before or after the frost dates.

So what are those dates? When can we expect the last frosts of spring and the first frosts of fall?

Unfortunately, there's no absolutely reliable way to tell when the nights are likely to drop below the freezing point. Gardeners are forced to rely on experience and guesswork. It's possible to make an educated guess with the following information, though.

For this part of Pennsylvania, it is sometimes said that the expected dates for first and last frosts are April 27th and October 15th, respectively. This would result in a frost-free growing season of about 171 days. However: those dates are the statistical 90th percentiles for hard freezes only; nine years out of ten, the last will arrive before/after those dates, but afterwards 10% of the time. It's also quite likely that there will be brief, light frosts capable of killing the more cold-tender plants long after these periods, and nights won't be warm enough for transplants or sown seeds of warmth-loving species to thrive for quite some time. The final frost of spring is likely to occur some time between April 30th-May 30th, and the first frost of autumn September 15th-October 15th.

June 6th is the traditional date to plant beans in Pennsylvania; it takes that long into the season for there to be no real chance of cold air. Occasionally there will be a light frost almost that late in the year, as the Community Garden found out in 2009 - a frosty night on the first of June caused damage to tomato plants in certain parts of the garden, although other locations just a few feet away seemed to be untouched.

It can be tempting to set out and sown plants with long growing periods as early as possible, but keep in mind that even if a seedling survives chilly temperatures, it will have to draw on its supplies of stored energy to do so, draining the resources it would otherwise use to grow. The benefit of the extra time can be offset by the stress of coping with adverse conditions; later-sown plants can outpace those that were too early, and be stronger and healthier too. Think carefully before taking advantage of an early heat wave, or before trying to establish warmth-loving plants in cool weather. You may regret the impulse.